Humidifier tub base



FIG. 1 is a perspective view of a humidifier tub base according to our design;

FIG. 2 is a top view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a left side view thereof; and,

FIG. 6 is a right side view thereof. 

The ornamental design for a humidifier tub base, as shown. 